1. Field of the Invention
This invention relates to the design of microwave cylindrical TE.sub.011 filters and specifically to the design of such filters utilizing TE.sub.211 /TE.sub.311 mode control for the purpose of improving filter selectivity and/or placing a transmission null at a desired frequency.
2. Description of the Prior Art
Previous efforts have been made to control the response characteristics of microwave TE.sub.011 resonators. Cavity shaping as a means to control the resonator response was reported by Herbert L. Thal, Jr. in IEEE Transactions Microwave Theory and Techniques. Vol. MTT-27, No. 12, Dec. 1979 at pages 982 to 986, a copy of which accompanies this application for a patent. That same article provides good background information for the subject of this application and it is hereby incorporated herein in full by this reference. Thal observed that in spherical cavities and in cylindrical cavities the TE.sub.011 mode was accompanied by degenerate modes at the same frequency. Thal suggested that there should exist intermediate cavity shapes, i.e., intermediate a cylinder and a sphere, which would isolate the desired cylindrical TE.sub.011 mode from the degeneracies that exist in the cylindrical and spherical cases.
While Thal's paper is primarily concerned with investigating the shape of the cavity, he does discuss,
though only briefly, the suppression of the TE.sub.211 /TE.sub.311 mode effects by careful selection of the angular displacement between the input port and the output port. He concludes that a displacement of 144 degrees (.theta.=36 degrees in FIG. 9 of Thal) is the approximate optimum angle to simultaneously suppress not only the TE.sub.211 and TE.sub.311 coupling, but the residual nonresonant TE.sub.211 /TE.sub.311 pattern as well.